1. Field of the Invention
The present invention relates in general to the measurement of speed of sound in a gas mixture. More particularly, the present invention relates to applications of ultrasound detectors for example in the measurement of gas concentration or gas flow in environments where pressure induced temperature influence is a dominant factor for measurement accuracy.
2. Description of the Prior Art
In some applications of ultrasound detection, such as the measurement of concentration or proportions of gas components in a gas mixture, pressure variations in the gas mixture has a large influence on the measurement accuracy. There is a well known connection between the speed of sound and gas specific parameters, and, based on this connection and measurement values of the speed of sound, a current proportion of gas content can be calculated. Furthermore, the speed of sound in a gas has a strong dependency on temperature, and in order to correctly calculate the gas content a timely and accurate temperature measurement on the gas is also required.
In medical breathing apparatuses it is of vital importance that gas proportions in for example inspiration or expiration air from a patient is accurately determined for the purpose of monitoring and controlling the dosing of gas components in a gas mixture or for monitoring the health state of a patient. However, when a patient is breathing, significant changes in pressure occur in the breathing apparatus and consequently, in accordance with the ideal gas law, the temperature of the gas varies largely. Thus, in order to accurately determine the proportions of the gas content it is generally required that a very precise temperature measurement is carried out, and that the temperature measurement and the sound speed measurement are carried out closely in time such that they describe the momentary physical state of the gas.
When measuring temperature in actual practice there is always a certain degree of delay in the temperature measurement in relation to the real current temperature. The delay depends on the time constant of the temperature sensor that is used for the measurement. The delay in the ultrasound sensor depends on the sampling frequency and is in general so short that it is insignificant and negligible in comparison with the temperature measurement. A concurrent measurement of sound speed and temperature will therefore always result in a, to some degree, erroneous temperature that in its turn causes an erroneously calculated gas concentration.
A number of different approaches are known to deal with this measurement problem. Examples are described in the following publications, which are all incorporated by reference in the present application.
In the technical report A SONAR BASED TECHNIQUE FOR THE RATIOMETRIC DETERMINATION OF BINARY GAS MIXTURES, G. Hallewell et al, Nuclear Instruments and Methods in Physics Research A264 (1988) 219-234, North-Holland, Amsterdam, there is a theoretical background to this kind of measurement.
U.S. Pat. No. 6,202,468 discloses an apparatus and a method for determining the relative proportions of gases in a mixture by measuring magnetic susceptibility and speed of sound.
U.S. Pat. No. 4,155,246 discloses a gas analyzing system using sonic wave shift over a tubular gas column. U.S. Pat. No. 4,932,255 discloses a method and device for measuring on a gas flow using surface acoustic waves over a substrate positioned in the gas flow. Thermally conductive paths around the substrate periphery reduce thermal gradients. In this piece of prior art the thermally conductive paths are devised in order to reduce thermal gradients that are created by the sensor in connection with transmission of surface acoustic waves.
U.S. Pat. No. 5,351,522 discloses an ultrasound detection based gas sensor with an L-shaped measurement chamber. This piece of prior art is directed to the problem of minimizing standing sound waves in the measurement chamber.
JP 2002 257 801 discloses an ultrasonic gas analysis sensor which deals with the problems of avoiding effects on the sound waves due to gas flow rate and diffusion. A measurement chamber with diffusion holes is positioned in a gas passage tube with a gas inlet and outlet.
EP 1 083 427 discloses a method for determining the gas content of for example oxygen in breathing gas be means of measuring speed of sound. Problems caused by temperature variations are dealt with by synchronising sound speed detection with one or a plurality of specific times in a respiratory cycle.
EP 1 336 841 discloses a method for determining the temperature in an acoustic gas meter by means of an elongate resistive temperature sensor positioned in the ultrasound propagation region of the gas meter.
GB 2 195 767 discloses concentration measurement of a substance, such as a liquid, using ultrasonic pulses and detection of an nth echo.
U.S. Pat. No. 5,060,506 discloses a method and an apparatus for measuring the ratio of gases in a two gas mixture such as a therapeutic oxygen/nitrogen mixture. The gas mixture is passed through a sample tube within which ultrasound waves travel in successive bursts of pulses at the resonant frequency of the transmitter/receiver pair. Between bursts is a quiescent time period having a duration that is long enough to assure dissipation of transients so that standing waves do not form. The delay caused by the transit time of the sound through the gas sample generates electrical pulses that are translated into an analogue signal which is then temperature-corrected. The resulting voltage is proportional to the transit time and thus to the gas composition. The sample tube is contained inside a larger cylindrical body to enhance the gas flow and provide thermal insulation which is intended to improve the accuracy of temperature compensation.
U.S. Pat. No. 4,938,066 discloses a method and apparatus dealing with the problem of length expansion in an acoustic sensor with an ultrasound transducer emitting sound pulses that are reflected against a proximate surface of an invar rod and against a second surface at the distal end of the invar rod. The time difference between detection of reflected pulses from the respective surfaces and the known length of the rod are used to calculate the speed of sound.
U.S. Pat. No. 6,481,288 discloses a method and apparatus for measuring the speed of sound employing a spherical measurement chamber.